It's not directly about "walking" Prada, it's more like Prada is Russel Marsh's (most important casting director) most selective show, so walking a Prada show can (!) open new doors if you know what i mean. Marloes Horst for exmaple says that clients take a look twice on her book since she was a prada exclusive in ss09.

Okay, I checked up and Natasha walked every Prada Show since Spring Summer 2008.
I wonder why she made her debut so late

That is a perfect example of why that whole "well she doesn't fit the line, she doesn't fit the line" talk is BS. "It" girls are cast for everything in sight, after that it's just a matter of who the casting director/designer likes (has good relationships with).

I'm thinking that, along with some of the obvious reasons (racial prejudice, not having the right "look", etc), there could be some slightly more creative reasons these big time models haven't walked in big time shows. For example, Michael Kors once told W magazine that he didn't book two hot new faces in the early nineties because one was too tall and "there was no way the sleeves would be long enough," and the other was far too short. Those new faces were Claudia Schiffer and Kate Moss, respectively. I'm thinking this could explain the long-ish list of shows that Irina and Snejana haven't walked in. And, of course, there are the girls that haven't walked in shows by choice, like the McQueen walkouts.

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"I say, let's have happy clothes. You could reply that's frivolous in this troubled world, but do you really think dressing like an existential nun with suicidal thoughts is going to solve Bosnia?"